In applications involving the interaction of a radiation beam with an optical storage medium, severe requirements are placed on the radiation detectors used in the focusing on and the tracking of information on the disk. In FIG. 1, a block diagram of the system for optical interaction with a storage medium in which the present invention can be used advantageously is shown. A current source 11 provides a current, I.sub.Laser, to laser diode 12. The radiation from laser diode 12 is reflected from the storage medium 13, typically an optical disk, and detected by radiation detector diode 14. The (radiation) detector (diode) 14 controls the current through measuring device 15. The detector 14 is typically comprised of a plurality of individual radiation sensitive diodes A, B, . . . N. For purposes of discussion, only two diodes A and B will be described, the extension to more than two diodes will be apparent. In FIG. 2A and FIG. 2B, the definitions of the critical parameters of the detector 14 output current are illustrated. In FIG. 2A, the radiation pulse applied to the detector rises from a `read` power level of 0.2 mw of power to approximately 2 mw of `write` power. The pulse lasts for approximately 90 ns. As shown in FIG. 2B, in response to the pulse of radiation, the detector output current rises from an initial value of current, I.sub.Read, to the maximum current value I.sub.Write. In order to simplify the discussion, the value of the `read` current in FIG. 2B has been taken to be normalized to be zero. Therefore, the current rise is I'.sub.Max =(I.sub.Write -I.sub.Read). (Note that I.sub.Read has been normalized and is in fact equal to zero.) The output current rise time, T.sub.r, is defined and the time required for the output current to rise from a value of 10% of I'.sub.Max to 90% of I.sub.Max when the pulse of radiation is applied thereto. The rise time, T.sub.r, is shown in FIG. 2B. The fall time, T.sub.f, is defined as the time for the output current from the radiation detector to fall from 90% of I'.sub.Max to 10% of I'.sub.Max when the radiation is removed from the radiation detector. The settling time, T.sub.ss, is defined as the time in which the output current of the radiation detector falls from 10% of I'.sub.Max to .+-.1% of I'.sub.Max with respect to `read` current value. Finally, the critical time parameter, T.sub.crit, is defined as the period of time after the completion of the T.sub.ss period in which the output current must remain .+-.1% of I.sub.Read with respect to zero radiation detector output read current value (i.e., I.sub.Read). The fall time, T.sub.f, the settling time, T.sub.ss, and the critical, T.sub.crit, are also illustrated in FIG. 2B.
The requirements for certain optical storage and retrieval applications are that the fall time, T.sub.f, should be approximately 10 ns. More importantly, the settling times, T.sub.ss, must have a value much less than the 10 ns fall time, T.sub.f. The positional accuracy for focusing and tracking of the information track require that the servo systems respond to a 1% deviation of the best focusing and/or tracking signals. The storage system responses to read and write operations require time frames of the order of 10 ns or less. With the radiation detectors currently available and with laser wavelengths of 780-830 nm, tail currents as deep as 20-45 .mu.m in the epitaxial layer or bulk contribute adversely to the settling time. In many implementations, fall times, T.sub.f, 10-15 ns are achieved with 20-45 ns of additional settling time caused by the tail currents within the non-active regions and deep within the vertical structure (in the epitaxial layer or in the substrate at vertical depths greater than the active regions).
A need has therefore been felt for a vertical structure designed to achieve the goal of settling times in the order of 5-8 ns in order for detectors to achieve fall times of 10 ns (i.e., the ratio of settling time to fall time &lt;1.0). This invention achieves this goal by incorporating into the detector structure a suction diode which will "dump" tail currents to ground rather than allow slow diffusion currents to persist. The suction diode surrounds the active detector regions, all sides as well as the entire underside of the vertical structure.